In electric controlled thermostatic mixing valves it is known to implement a thermistor type temperature sensing element as taught for instance in EP1229418 to Mountford at al or in WO2009072049 to the present applicant. Thermistors are actually variable resistors which change their resistance according to the surrounding temperature, they are easy to implement and stable on reading.
Thermistors may accept very small size but they are sensitive to the environment and must be electrically and mechanically isolated from the measured medium in particular when the medium is liquid. It is common practice to isolate such thermistors with direct glass or epoxy coating and encapsulate the fragile assembly in a brass or copper probe. The protective probe as well as the electric insulation, significantly extends the response time of the sensor. The response time is commonly expressed in terms of thermal time constant which is the time it takes for the sensor to reach 63.2% of the change in temperature. An example of such encapsulated thermistor commercial model is: FRP5 Fast-Response Thermistor Sensor made by ATC Semitec Ltd. In U.K. The time constant of such thermistor according to its data sheet is about 1 sec.
The sensor response time in an electrically operated mixing valves is of critical importance for stable temperature control especially during low flow rate conditions, hence the use of a thermistor may limit the overall performance of the control valve.
Thermocouple assemblies are in use since 1821, when T. J. Seebeck discovered the thermoelectric effect. According to the Seebeck effect, thermocouple circuit made of two wires of dissimilar metals connected at both ends (junctions), will generate a current and measurable, low voltage output that is approximately proportional to the temperature difference between the hot junction and the cold junction. Thermocouples are generally more rugged than thermistors of the same size but they require precise high gain electric amplification to yield a usable signal.
Three typical constructions of thermocouple assemblies are in common use, listed here by order of the response time; the ungrounded type where the sensing junction is enclosed in a sheath or probe and the junction is electrically isolated from the probe wall. The grounded type where the sensing junction, is wired directly to the probe wall, and the exposed type where the junction is outside the probe wall and is in direct contact with the target medium. The exposed type possesses the best heat transfer and quickest response time typically more than 50 times faster than an equivalent size sheathed thermistor probe. However the exposed thermocouple is also limited to non corrosive, typically dry surrounding medium.
In order to achieve fast response time close to that of an exposed junction while maintaining acceptable sealing and protection of the junction, it was suggested in U.S. Pat. No. 2,466,175 to Kretsch et al to use a coaxial thermocouple of which the first less corrosive metal is in the form of a tube which encircles the second more corrosive dissimilar metal accepting the form of a wire. The tube and the wire are spaced apart by an insulating media and welded together at one end to form a sealed junction.
A draw back of such construction lies in the fact that the outer metal tube has relatively large cross-section compared to the wire, allowing heat conduction to or from the region of the tube that is not immersed in the surrounding medium. This thermal wicking effect increases the thermal time constant of the thermocouple assembly. For instance, if a fast change of the medium temperature occurs such as happens in flows of hot and cold water in a mixing valve, response time will increase due to the fact that part of the heat energy is being dissipated along the thermocouple external metal tube to a portion that is far from the measuring junction.
Reducing the wall thickness of the metal tube or increasing the length of the immersed portion may improve performance, however such temperature sensors as indicated above for water mixing applications, typically protrude from the wall of the flow pattern towards the center of the flow and are subject to bending stresses formed by the impact of water which can gain speeds of up to 30 m/s. Certain minimal wall thickness and maximal exposure length must be maintained for reliable operation.
Another solution for fast response thermocouple is suggested in U.S. Pat. No. 7,004,626 to Giberson et al. Giberson describes a small mass hollow tip formed of a first-metal having a wire of the same metal welded internally to the shoulder of the tip and a second wire of a dissimilar metal extending into the hollow tip and through a small hole in the closed end of the tip. The second wire, protruding through the small hole, is welded at the outside of the tip. For mechanical strength, the hollow tip is held in an open ended sheath by a packing of thermal insulating material and RTV bond. This solution may serve its purpose but is expensive in production and may fail to properly seal the assembly against moister along an extended period of time.
Accordingly, there is a need for a fast responding, thin thermocouple assembly of the coaxial type, that is suitable for temperature measurement of fast flowing liquid streams. The ease of assembly and proper sealing of the junction and the external housing are of great importance.